Here are a few of my favorite entries, all beginning with the letter “J," from Word and Phrase Origins, a thoroughly delightful book published in 1997.
Jack Frost. Jack Frost has been the personification of frost or cold weather since 1826, when the term was first used in a British sporting magazine.
Jargon. Jargon (or jargoun) meant "meaningless chatter" in Medieval French. It is now the special vocabulary of a trade or profession, which sounds like meaningless chatter to the outsider.
Jerky. The word jerky, first recorded in 1850, refers to smoked strips of dried beef. It is the Anglicization of the Spanish word charqui (dried meat).
Josh. The best guess is that josh, in its meaning of "to kid" or "to fool around" is a merging joke and bosh (the latter meaning "nonsense"). The term dates back to the mid-19th century.
July. Mark Antony named the seventh month of the year in honor of Julius Caesar, as it was his birth month. Up until about 1800, July was pronounced in English like the girl's name Julie.
A note about the author: Word and Phrase Origins was written by Robert Hendrickson, the author of more than 25 other books, including American Literary Anecdotes, New York Tawk, and More Cunning than Man: A Social History of Rats and Men.